1. Field of the Invention
The invention relates to electronic packaging, more specifically it relates to a method of electrolessly plating nickel on an anodized aluminum substrate with subsequent direct attachment of chips thereto.
2. Description of the Prior Art
Commonly assigned U.S. Pat. No. 4,431,707 relates to plating anodized aluminum substrates, in particular a method of preparing aluminum substrates for utilization as printed circuit boards.
A number of electroless plating formulations are commercially available. These formulations are diverse in their chemical compositions, reactivity, and operating conditions. Most, however, require high operating temperatures and are either acid based, having a pH in the range of 1-4, or alkaline based, with a pH in the range of 10-13. Either of these conditions are detrimental to an anodic coating on aluminum because they either react with and/or solubilize the surface layer.
As a result of these detrimental effects, problems may arise such as unsatisfactory adhesion characteristics, anodic coating thinning to the point where its voltage breakdown is compromised and, in the case of additive plating, plating over the entire surface, which causes shorts and renders the coating unacceptable for packaging applications.
Anodized aluminum substrates have excellent thermal dissipation capabilities, may have a built-in ground plane and lend themselves to a wide variety of applications commonly assigned U.S. Pat. Nos. 4,894,126 and 4,898,651 relate to anodized aluminum substrates which are useful in electronic packaging applications.
Because of the just noted advantages incident to the use of anodized aluminum substrates as printed circuit substrates, it is desirable to be able to directly attach electronic chips to an appropriately circuitized anodized aluminum substrate as this is a direction the industry is pursuing. When high count I/O chips are mounted on carrier substrates, concern arises about manufacturability, thermal management and long term reliability. These are considerations of particular concern when the directly attached device consumes high power.
It is known to use polymer coated metal carriers for receiving directly attached chips, where the attachment is made through wire bonding or similar attachment methods. Inherent problems associated with such a manufacturing process relate to adhesion and inadequate heat dissipation characteristics.